The invention relates to a process for the recycling of material comprising unhardened prepreg wastes to produce fiber-containing press compounds (BMC).
Prepregs are fiber products in sheet form or in strand form which comprise glass fibers or carbon fibers and which have been treated with reactive resin. Resin used can be epoxy resin. The epoxy resins used are formulated in such a way that they harden at a particular temperature. Epoxy resins used in aircraft construction mostly harden at 180° C.; curing temperatures of the epoxy resins used in other technical sectors are usually different, mostly being significantly lower. Because the reactive resin of the prepregs has limited shelf life, it can be necessary to cool prepregs during storage. After a particular period of exposure of these prepregs to higher temperatures (for example room temperature) they are subject to usage limitations, because the resin has already reacted to an unacceptable extent, and cures. It is naturally desirable that this material that has exceeded its shelf life is recycled. Carbon prepregs in particular are, moreover, very expensive.
Lay-up procedures, for example AFP processes or ATL processes (automated fiber placement, automated tape laying) moreover produce offcut material. On the prepreg rolls there can, moreover, be prepreg residues that cannot be used for lay-up (roll ends). The final cutting-to-size of a workpiece prior to hardening can also produce single- or multilayer wastes.
Fiber-containing press compounds are composed of short glass fibers or short carbon fibers and a matrix resin, and are known as bulk molding compounds (BMC). BMC can be processed in the hot-press process. For this, the BMC is inserted centrally into a heated, divided mold. Closure leads to distribution of the BMC within the mold cavity. BMC can also be processed by injection molding.
BMC can be composed of a mixture of from 10 to 35% of resin, from 10 to 50% of fibers and from 0 to 70% of fillers and/or additives.
Production of BMC from prepreg wastes can begin with removal of the resin, for example by pyrolysis. The resin-free fibers can then be comminuted and mixed with fresh resin, fillers and additives.
DE 19514543 C1 discloses a process for the reclamation and recycling of offcut material wastes from webs made of resin that is still reactive, where the offcut material wastes are chopped in a particular way, the viscosity of the resin is reduced by heating or addition of solvent, and mechanical shear stress is used to produce a mixture of separated fibers and resin which is then used as raw material for further processing.